The present invention relates to a direct conversion type of a small wireless radio which does not use any SAW (surface acoustic wave) filter having a steep cut-off frequency characteristic.
Conventionally, in a direct conversion type of a reception circuit of a small wireless radio, as shown in FIG. 5, an FSK (frequency shift keying) modulation signal 1 received through an antenna 2 is subjected to orthogonal conversion by means of mixers 5a and 5b with second local oscillation signals which are obtained directly and through a phase shifter 16A from the frequency F1 of the local oscillation signal generated by a multiplier 10. A synthesizer circuit 9 constituted by a VCO 28, a PLLIC 26 and an LPF 27 produces an output of a frequency F.sub.v, and this output frequency F.sub.v is then multiplied by N by the multiplier 10. The multiplier outputs the signal of the frequency N.multidot.F.sub.V which is the above-mentioned second oscillation signal of the frequency F1. The frequency bands of the thus obtained base band signals are limited through channel filters 14a and 14b and limiter amplifiers 15a and 15b respectively. The frequency of an FSK wave obtained by an orthogonal modulator 18 is detected by a detector 21. The reference numeral 16B designates a phase shifter. In the drawing, reference numerals 16A and 16b denote phase shifters, and numeral 17 denotes an oscillator which generates a rectangular oscillation signal.
The average voltage of the frequency-to-voltage relational characteristic (hereinafter referred to as an F/V curve) obtained on the basis of the detection output is compared with a reference voltage by a frequency drift detector 22. An AFC control signal 23 corresponding to the quantity of the frequency deviation is sent to a reference crystal oscillation circuit portion 8 so that the oscillation frequency of a crystal oscillation circuit 25 is controlled through a frequency control portion 24.
Here, the reception frequency of the FSK modulation signal 1 is presented by f.sub.0, the oscillation frequency of the VCO 28 is presented F.sub.V, the value of multiplication factor of the multiplier is presented by N, and the frequency of the local oscillation signal is presented by F1. Then, the following expression (equation 1) is established because of the direct conversion.
f.sub.0 =F1=N.multidot.F.sub.V PA1 N=f.sub.0 /F.sub.V
It is found that the reception frequency f.sub.0 is equal to the local oscillation frequency F1, and the value of N cannot be reduced unless F.sub.V is increased.
In the foregoing conventional receiver of the direct conversion type, since the reception frequency f.sub.0 is equal to the local oscillation frequency F1, it is necessary to strictly shield the local oscillation circuit with a shielding plate to prevent the local oscillation frequency from radiating as an interfere wave against the receiver per se so as not to lower the reception characteristic.
Further, in order to reduce the value of the multiplication factor N of the multiplier, it was necessary to change the design so as to increase the frequency of the VCO per se.
Thus, there has been such a problem that the design of the local oscillation circuit is difficult.